1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus using an electrophotographic process such as an image forming apparatus provided in a digital full color copying machine using an electrophotographic process, comprising means for automatically adjusting an image density of an image to be reproduced so as to obtain a desirable proper image density thereof.
2. Description of the Related Art
Conventionally, there have been put into practical use various kinds of electrophotographic image forming apparatuses such as a laser printer for driving a laser diode based on digital image data of an image of an original and reproducing the image of the original on a sheet of printing paper. Further, there have been proposed various kinds of digital image forming methods for faithfully reproducing a half-tone image such as a photograph.
As the digital image forming methods of these type, there have been known to those skilled in the art, an area gradation method using a dither matrix, and multi-value laser exposure methods such as a pulse width modulation method for representing a gradation of one dot image to be printed by changing a pulse width or an emitting time of a beam of laser light so as to change a light amount thereof defined as a product of the emitting time and an emitting intensity, and an intensity modulation method for representing a gradation of one dot image to be printed by changing an emitting intensity of a beam of laser light so as to change a light amount thereof (See Japanese Patent Laid-open Publication Nos. 62-91077, 62-39972, 62-188562 and 61-22597). Further, there has been publicly known a multi-value dither method which is a combination of the dither method and the above-mentioned pulse width modulation method or the above-mentioned intensity modulation method.
In the gradation method of this type for representing a gradation, it is considered possible in principle to reproduce an image density having a gradation strictly corresponding to a gradation of image data to be reproduced, however, an original density to be reproduced is not correctly proportional to an actually reproduced image density (referred to as an image density hereinafter) because of a complicated combination of characteristics of a photoconductor and toners and circumstances etc. In other words, a relationship between the image density and the original density shows a characteristic curve B which is shifted from a characteristic curve A to be originally obtained, as schematically shown in FIG. 4. Such characteristic as above is generally called a .gamma. characteristic, which mainly causes deterioration of faithfulness of reproduced images of originals, particularly of a half-tone original.
Therefore, in order to improve faithfulness of a reproduced image, conventionally, there has been performed a so-called .gamma. correction process for converting data of a read original density into data using a predetermined .gamma. correction table and forming a digital image of dot images based on the converted data of the original density so that the relationship between the original density and the image density becomes linear, namely, the above-mentioned linear characteristic A shown in FIG. 4 can be obtained. Thus, normally, the image of the original can be faithfully reproduced depending on the original density by performing the above-mentioned the .gamma. correction process.
On the other hand, as one of phenomena due to another cause for influencing the image density, there is such a phenomenon that an adhering amount of toner onto the photoconductor changes upon a developing process using the toner when characteristics of the photoconductor and the toner changes due to change in external circumstances such as the temperature, the humidity, etc. Generally speaking, the adhering amount of toner increases under circumstances of a high temperature and a high humidity so that the original image having a higher image density is reproduced with a .gamma. characteristic having a relatively large gradient in a relatively high original density. On the other hand, the adhering amount of toner decreases under circumstances of a low temperature and a low humidity so that the original image having a lower image density is reproduced with a .gamma. characteristic having a relatively small gradient in relatively low and middle original densities.
Thus, there is such a problem that the reproduced image density changes due to change in the circumstances. In order to solve the above-mentioned problem so as to obtain a stable proper image density, there has been performed an image density control process for controlling the maximum image density to be constant, generally, in a conventional electrophotographic copying machine, a conventional electrophotographic printer, or the like.
One of the above-mentioned image density control processes which has been put into practical use will be described below with reference to FIG. 5 for illustrating an image forming part comprising a photoconductive drum 41 and a developing roller 45r.
Referring to FIG. 5, a corona charger 43 having a discharging electric potential V.sub.C is provided so as to confront a photoconductive drum 41. A grid voltage V.sub.G is applied to a grid of the corona charger 43 by a grid voltage V.sub.G generator 214. An electric potential Vo on the surface of the photoconductive drum 41 is controlled by changing the grid voltage V.sub.C based on the electric potential Vo detected by a Vo sensor 44.
In the first place, prior to an exposure of a beam of laser light, a negative surface electric potential Vo is applied to the photoconductive drum 41 by the corona charger 43, and a negative developing bias voltage V.sub.B (.vertline.Vo.vertline.&gt;.vertline.V.sub.B .vertline.) of a relatively low electric potential is applied to the developing roller 45r by a developing bias voltage V.sub.B generator 215 in order to prevent a fog. In this case, the surface electric potential of a developing sleeve of the developing device is also set to the developing bias voltage V.sub.B.
The surface electric potential Vo of the photoconductive drum 41 changes upon an exposure of a beam of laser light (referred to as a light exposure hereinafter) into an electrostatic latent image electric potential V.sub.I upon the light exposure with the maximum light amount of the laser light. When the electrostatic latent image electric potential V.sub.L becomes lower than the developing bias voltage V.sub.B, the toner adheres onto the photoconductive drum 41. The adhering amount of toner increases as a difference between the developing bias voltage V.sub.B and electrostatic latent image electric potential V.sub.L becomes larger. Therefore, since the difference between the developing bias voltage V.sub.B and electrostatic latent image electric potential V.sub.L is changed by changing the surface electric potential Vo on the photoconductive drum 41 and the developing bias voltage V.sub.B, the adhering amount of toner onto the photoconductive drum 41 can be changed, thereby eventually controlling the image density of the toner image.
According to the image density control process of this type as described above, the maximum image density is made constant by automatically or manually by an operator's changing the surface electric potential Vo on the photoconductive drum 41 and/or the developing bias voltage V.sub.B.
In the automatic image density control process, a reference toner image of a reference image pattern which becomes a reference for the image density control process is formed on the surface of the photoconductive drum 41, and a light amount of a reflected light from the reference toner image is detected by an automatic image density controlling sensor (referred to as an AIDC sensor hereinafter) 203 provided in the vicinity of the photoconductive drum 41. Data of the detection value detected by the AIDC sensor 203 are inputted to a printer controller 201, which in turn controls the grid voltage V.sub.G generator 214 and the developing bias voltage V.sub.B generator 215 in accordance with a comparison result between the data of the detection value detected by the AIDC sensor 203 and a predetermined value. The above-mentioned process is repeated until the adhering amount of toner becomes the predetermined value.
However, even through the image density control process is performed so as to make the image density constant on the basis of the output of the AIDC sensor 203, a relationship between the output of the AIDC sensor 203 and the actual image density of the reference toner image changes due to change in characteristics of the AIDC sensor 203 and the circumstances from those in the initial state when the digital full color copying machine is manufactured. Therefore, the automatic image density control process can not be stably performed.